Angiotens II receptors: Brain angiotens II receptors have been characterized as two distinct subtypes: AT1 and AT2. Each subtype is heterogenous. They occur as early as embryonic day 18, develop differently and have differential guanine nucleotide and pertussis toxin sensitivities. Cerebral artery angiotensin II receptors are of the AT2 type. Brain, cerebral artery, and pituitary angiotensins receptors vary during the estrous cycle and are regulated by estrogens. Rat sympathetic ganglia contain AT1 receptors coupled to PI turnover. Large numbers of AT2, and smaller amounts of AT1, receptors are present in fetal tissues. Fetal AT1, but not AT2, receptors are coupled to PI turnover. A major proportion of angiotensin receptors in aortas from rat fetus and immature rats are of the AT2 subtype. Angiotensin II receptors in the rat spleen are of the AT1 subtype. Melatonin receptors: Cerebral and caudal artery melatonin receptors vary during the estrous cycle and are regulated by estrogens. Arterial melatonin receptors are lower in hypertensive rats. Atrial natriuretic peptide receptors: There are different subtypes in the choroid plexus and subfornical organ. C-atrial natriuretic peptide receptors are very selectively localized in the rat brain. These receptors are very high in fetal limbic system cortex. Insulin-like growth factor (IGF-I) receptors. IGF-I receptor development was characterized in rat and hamster hypothalamic nuclei and pituitary gland. Both thyroid hormones and estrogen regulate IGF-I receptors in rat hypothalamic nuclei.